1. Field of the Invention
The present invention relates to a thin film transistor substrate for a liquid crystal display panel, and more particularly to a thin film transistor substrate of horizontal electric field type and a method of darkening a defective pixel in the same.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) device controls the light transmittance of a liquid crystal material by applying an electric field to thereby display a picture. Liquid crystal displays are largely classified into vertical electric field type and horizontal electric field type, depending upon the direction of the applied electric field.
A liquid crystal display of a vertical electric field type drives a liquid crystal in a twisted nematic (TN) mode by applying a vertical electric field to the liquid crystal material. The vertical electric field is formed between a pixel electrode and a common electrode arranged opposite to each other on an upper substrate and lower substrate, respectively. The liquid crystal display of the vertical electric field type provides a large aperture ratio, but has a narrow viewing angle of about 90°.
The liquid crystal display of the horizontal electric field type drives a liquid crystal in an in plane switching (IPS) mode by applying a horizontal electric field to the liquid crystal material. The horizontal electric field is formed between the pixel electrode and the common electrode arranged in parallel to each other on a lower substrate. The liquid crystal display of the horizontal electric field type provides a wide viewing angle of about 160°. More specifically, the liquid crystal display of the horizontal electric field type includes a thin film transistor substrate (i.e., lower substrate) and a color filter array substrate (i.e., upper substrate). The thin film transistor substrate and the color filter array substrate are joined opposite to each other. A spacer is provided to maintain a uniform cell gap between two substrates. The cell gap is filled with a liquid crystal material.
The thin film transistor substrate is comprised of a plurality of signal wires for forming a horizontal electric field for each pixel, a plurality of thin film transistors, and an alignment film coated thereon to align the liquid crystal. The color filter array substrate includes a color filter for color generation, a black matrix for preventing light leakage, and an alignment film coated thereon to align the liquid crystal material.
FIG. 1 is a plan view showing a structure of a related art thin film transistor substrate of a horizontal electric field type. Referring to FIG. 1, the thin film transistor substrate of the horizontal electric field type includes a gate line 2 and a data line 4. A gate signal is applied to the gate line 2 and a data signal is applied to the data line 4. A gate insulating film (not shown) is provided between the gate line 2 and the data line 4.
A thin film transistor 6 is provided at a crossing of the gate line 2 and the data line 4. A pixel region is defined by the crossing of the gate line 2 and the data line 4. A pixel electrode 18 and a common electrode 20 are provided in the pixel area for forming a horizontal electric field. A common line 16 is connected to the common electrode 20. The common line 16 supplies a reference voltage for driving the liquid crystal. The common line 16 is provided in parallel to the gate line 2 with the pixel area therebetween. The common line 16 is adjacent to a next-stage gate line 2.
The thin film transistor 6 allows a pixel signal of the data line 4 to be charged and maintained on the pixel electrode 18 in response to the gate signal from the gate line 2. To this end, the thin film transistor 6 includes a gate electrode 8, a source electrode 10, a drain electrode 12, and an active layer 14. The gate electrode 8 is connected to the gate line 2. The source electrode 10 is connected to the data line 4. The drain electrode 12 is connected to the pixel electrode 18 in opposition to the source electrode 10. The drain electrode 12 further includes an extended part 12A, which extends in parallel to the gate line 2. The extended part 12A of the drain electrode 12 is provided in opposition to a horizontal part 20B of the common electrode 20 with the gate insulating film (not shown) therebetween to form a first storage capacitor.
An active layer 14 overlaps the gate electrode 8 including the gate insulating film (not shown) therebetween to define a channel between the source electrode 10 and the drain electrode 12. The active layer 14 also overlaps the data line 4. On the active layer 14, an ohmic contact layer (not shown) is provided to establish an ohmic contact with the data line 4, the source electrode 10 and the drain electrode 12.
The pixel electrode 18, which is provided in the pixel area, is connected to the drain electrode 12 of the thin film transistor 6 via a first contact hole 24 through a protective film (not shown). The pixel electrode 18 includes a first horizontal part 18A overlapping the extended part 12A of the drain electrode 12, and a finger part 18B extended from the first horizontal part 18A into the pixel area. The pixel electrode 18 further includes a second horizontal part 18C commonly connected to the finger part 18B thereof and overlapping the common line 16 in order to form a second storage capacitor.
An upper storage electrode 22 is further provided between the common line 16 and the second horizontal part 18C of the pixel electrode 18 to increase a capacitance of the second storage capacitor. The upper storage electrode 22 overlaps the common line 16 having the gate insulating film (not shown) therebetween while overlapping the second horizontal part 18C of the pixel electrode 18 including the protective film (not shown) therebetween. The upper storage electrode 22 is connected to the second horizontal part 18C of the pixel electrode 18 via a second contact hole 26 through the protective film.
The common electrode 20 is connected to the common line 16 and is provided in the pixel area to form a horizontal electric field along the pixel electrode 18. To this end, the common electrode 20 includes a finger part 20A provided in parallel to the finger part 18C of the pixel electrode 18. Tthe common electrode 20 further includes a horizontal part 20B that is commonly connected to the finger part 20A thereof and overlaps the extended part 12A of the drain electrode 12 including the gate insulating film therebetween.
In related art the thin film transistor substrate of horizontal electric field type, the source electrode 10 and the drain electrode 12 are formed through the same mask process for patterning a source/drain metal layer along with the data line 4 and the upper storage electrode 22. In this case, when a distance between the source electrode 10 and the drain electrode 12 is relatively small, patterns are malformed causing shorts between the source electrode 10 and the drain electrode 12 like the A portion.